NDIR gas sensor

ABSTRACT

A miniaturized NDIR gas sensor is manufactured using semiconductor micromachining techniques from a semiconductor material such as Si or GaAs. The NDIR gas sensor comprises an optical waveguide, a light source at one end of the waveguide, at least one light detector at the end of the waveguide opposite the light source, a diffusion type gas sample chamber formed within the waveguide and interposed in the optical path between the light source and light detector so that the light source and light detector are thermally isolated from the gas sample, and a separate bandpass filter interposed between the light source and each light detector. A miniaturized NDIR gas sensor is also provided in which the light source and light detector are located on the same end of the optical waveguide is also provided. Because the NDIR sensor is fabricated out of a semiconductor material, the source driver and signal processing electronics may be added directly to the sensor using integrated circuit fabrication techniques. Particles and smoke and dust may be kept out of the sample chamber by application of a gas permeable membrane over apertures in the sample chamber walls.

This application is a continuation of copending application Ser. No.08/662,981 filed Jun. 13, 1996, which is a continuation of applicationSer. No. 08/518,134 filed Aug. 22, 1995, now abandoned, which is acontinuation-in-part of application Ser. No. 08/284,914, filed Aug. 2,1994, now U.S. Pat. No. 5,444,249, which is a continuation ofapplication Ser. No. 08/195,523, filed Feb. 14, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the field of gas sensingdevices and, more particularly, to NDIR gas analyzers.

2. Description of the Prior Art

Nondispersive infrared (NDIR) gas analyzers are used for detecting thepresence and concentration of various gases. The NDIR technique has longbeen considered one of the best methods for gas measurement. In additionto being highly specific, the NDIR gas analyzers are also verysensitive, stable and easy to maintain.

In contrast to NDIR gas sensors, which are by definition noninteractive,interactive optical gas sensors are less reliable, are generallynonspecific, and in some cases can be poisoned into a nonfunctionalstate.

Interactive gas sensors are generally nonspecific because the reagentbeing used to determine the concentration of the desired gas may reactwith other gases that are present. This will naturally result in falsereadings. Further, if the equilibrium of the reaction between thenonspecific gas and the reagent is such that the gas and reagent remainreacted even after the partial pressure of the gas drops in theenvironment being monitored, the sensor will no longer function properlyand is poisoned.

The response time for NDIR gas sensors is also typically shorter thanthat for interactive gas sensors. The reason being that the kinetics ofthe reaction between the sample gas and reagent controls how quickly thesensor detects a change in the concentration of the gas in theenvironment being monitored.

Despite the fact that interactive gas sensors are unreliable and thatthe NDIR gas measurement technique is one of the best, NDIR gasanalyzers have not enjoyed wide spread application because of theircomplexity and high cost of implementation.

In the past, NDIR gas analyzers typically included an infrared source, amotor-driven mechanical chopper to modulate the source, a pump to pushor pull gas through a sample chamber, a narrow bandpass interferencefilter, a sensitive infrared detector plus expensive infrared optics andwindows to focus the infrared energy from the source onto the detector.

In an attempt to reduce the cost and simplify the implementation of theNDIR technique, a low-cost NDIR gas sensor technique was developed. Thelow-cost NDIR technique employs a diffusion-type gas sample chamber ofthe type disclosed in U.S. Pat. No. 5,163,332, issued Nov. 17, 1992, tothe present applicant, and hereby incorporated by reference. Thisdiffusion-type gas sample chamber eliminates the need for: expensiveoptics, mechanical choppers, and a pump for pushing or pulling the gasinto the sample chamber. As a result, a number of applications for theNDIR technique, which were previously considered impractical because ofcost and complexity, have been opened.

The diffusion-type gas sample chamber of U.S. Pat. No. 5,163,332 uses anelongated hollow tube having an inwardly-facing specularly-reflectivesurface that permits the tube to function as a light-pipe fortransmitting radiation from a source to a detector through the samplegas. A plurality of filtering apertures in the wall of the non-porushollow tube permit the sample gas to enter and exit freely under ambientpressure. Particles of smoke and dust of a size greater than 0.1 micronare kept out of the chamber by use of a semi-permeable membrane thatspans the apertures in the hollow tube, and condensation of the samplegas is prevented by heating the sample chamber electrically to atemperature above the dew point of the gas.

Although the low-cost NDIR gas sensor technique opened a wide variety ofnew applications, the gas sample chamber and the corresponding gassensor of the low-cost NDIR technique are still too large for manypotential gas sensor applications. As a result, applications in whichlow-cost NDIR gas sensors may be used remain limited. Furthermore, whilethe cost of gas sensors employing the gas sample chamber of U.S. Pat.No. 5,163,332 is less than previous NDIR gas sensors requiring expensiveoptics, pumps, and choppers, a further reduction in the cost of NDIR gassensors would further increase the number of applications in which suchsensors are used and the frequency of their use.

Therefore, while a need exists for a compact, inexpensive NDIR gassensor, this need has gone unfilled. Accordingly, a goal of the presentinvention is to further advance the NDIR technique by providing aminiaturized, reliable, and low cost NDIR gas sensor.

SUMMARY OF THE INVENTION

The present invention is directed to an NDIR gas sensor for detectingthe concentration of a predetermined gas. To this end, an opticalwaveguide is provided having a light source at one end and a lightdetector at the other end. A bandpass filter is interposed in theoptical path between the light source and detector, so that the detectorprimarily receives radiation of a wavelength that is strongly absorbedby the gas whose concentration is to be determined. The waveguide isformed from two or more substrates of a semiconductor material, at leastone of which has been micromachined. In addition, the light source anddetector are directly manufactured on at least one of the semiconductorsubstrates used to form the optical waveguide. Preferably, a pair ofwindows are also optically disposed between the light source anddetector so as to define therebetween a sample chamber within theoptical waveguide. The windows thermally isolate the light source andlight detector from the sample gas, thus preventing the sample gas fromcooling these elements. The gas whose concentration is to be determineddiffuses into and out of the sample chamber in the optical waveguidethrough apertures or slots in the semiconductor substrates.

Because the walls of the optical waveguide are reflective, radiation istransmitted from the light source to the light detector through thesample gas without the need for expensive optics. Furthermore, becausegas sensors according to the present invention employ a diffusion-typegas sample chamber, no pump is required to push or pull the sample gasinto the sample chamber.

In a preferred embodiment, a gas permeable dielectric layer is depositedover the apertures to act as a filter and prevent dust or smokeparticles from entering the optical waveguide sample chamber.Preferably, the gas permeable layer prevents particles larger than about0.1 μm from entering.

In another preferred embodiment, at least a portion of the opticalwaveguide is metallized thereby improving its internal reflectivity andthe overall efficiency of the NDIR gas sensor according to the presentinvention.

In yet another preferred aspect of the present invention, an NDIR gassensor is provided that prevents condensation of gases or vapors on thewalls of the sample chamber. To accomplish this object, means areprovided for heating the gas sample chamber so that its temperatureremains above the dew point of any gas or vapor that might have atendency to condense on an inner surface of the sample chamber.

Other integrated circuit semiconductor devices can also be addedwherever needed to further enhance the performance of the NDIR gassensor according to the present invention. For example, temperaturesensors, pressure transducers, and humidity sensors may be added. Inaddition, a micro-flow sensor may be added to detect the flow rate ofthe sample gas through the sample chamber.

In a particularly preferred embodiment of the present invention, an NDIRgas sensor is provided which can be used to simultaneously determine theconcentration of a plurality of gases in the gas sample. An NDIR gassensor according to this embodiment is comprised of a plurality ofdetectors and a plurality of bandpass filters. Each bandpass filter isinterposed in the optical path between the light source and one of theplurality of detectors. The number of gases whose concentration isdesired to be determined dictates the specific number of detectors andbandpass filters that are required. Each bandpass filter, therefore, isdesigned so that the detector it is associated with primarily receivesradiation of a wavelength that is strongly absorbed by the gas whoseconcentration that detector is to determine.

Alternatively, in this embodiment, at least one of the detectors may beused as a reference detector. In this situation, the bandpass filterinterposed in the optical path between the light source and thereference detector must be designed to pass a neutral wavelength. Inother words, the bandpass filter must pass a wavelength of light that isnot absorbed by the gas sample.

In another particularly preferred embodiment, an NDIR gas sensor isprovided wherein the light source and light detector are provided in aside by side relationship, both in optical communication with theopposite end of the optical waveguide, so that the sample chamber isinterposed in the optical path between the two. An advantage of thisrelationship as opposed to an NDIR gas sensor which has a straightoptical waveguide with a light source and a light detector provided atopposing ends is that the NDIR gas sensor of the present embodiment istwice as sensitive. Thus, the NDIR gas sensor of the present embodimentcan achieve the same sensitivity with a sample chamber half as long.

In yet another preferred embodiment of the present invention, an opticalwaveguide is employed that includes a sample chamber that extends alongan indirect path. As with the first embodiment, the light source andlight detector are located at opposite ends of the waveguide and a pairof windows are preferably disposed between the light source and detectorso as to define the sample chamber therebetween. Preferably the indirectpath traversed by the sample chamber has a serpentine or spiral shape.An advantage of the present embodiment is that a sample chamber having alength much longer than the length of the substrate in which it isformed can be produced, thus providing enhanced sensitivity withoutsubstantially increasing the overall dimensions of the NDIR gas sensor.

Accordingly, it is a primary object of the present invention to providea miniaturized NDIR gas sensor which is micromachined from asemiconductor material.

Further objects and advantages of the invention will be betterunderstood from the following description considered in connection withaccompanying drawings in which the preferred embodiments of theinvention are illustrated by way of example. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an oblique view of an NDIR gas sensor according to a firstembodiment of the present invention;

FIG. 2 is a an oblique view showing a partial cutaway of an NDIR gassensor according to a first embodiment of the present invention;

FIG. 3 is a longitudinal cross sectional view along the line 3--3 inFIG. 1;

FIG. 4 is simplified optical diagram showing the path of a ray ofradiation through the gas sample chamber;

FIG. 5 illustrates a black body light source for use in an NDIR gassensor according to the present invention;

Fig. 6 is a schematic diagram of a system using a gas sensor accordingto the present invention;

FIG. 7 is a plan view of a bottom substrate for fabricating a gas sensoraccording to a second embodiment;

FIG. 8 is a plan view of a bottom substrate for fabricating a gas sensoraccording to a third embodiment;

FIG. 9 is a plan view of a bottom substrate for fabricating a gas sensoraccording to a forth embodiment;

FIG. 10 is a plan view of a bottom substrate for fabricating a gassensor according to the third embodiment with additional performanceenhancing semiconductor devices fabricated thereon;

FIG. 11 is a longitudinal cross sectional view along line 11--11 in FIG.10, except that the cross section is taken through a complete NDIR gassensor according to the present invention rather than just the bottomsubstrate;

FIG. 12 is an oblique view of an NDIR gas sensor according to a fifthembodiment;

FIG. 13 is an oblique view showing a partial cutaway of an NDIR gassensor according to the fifth embodiment;

FIG. 14 is a longitudinal cross sectional view along line 14--14 in FIG.12;

FIG. 15 is plan view of a bottom substrate for fabricating an NDIR gassensor according to the fifth embodiment;

FIG. 16 is a schematic diagram of one possible extended sample chamberfor an NDIR gas sensor according to a sixth embodiment; and

FIG. 17 is a schematic diagram of a second possible extended, samplechamber for an NDIR gas sensor according to the sixth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention is described in connectionwith FIGS. 1-3. As shown in FIGS. 1-3, the NDIR gas sensor 28 accordingto the present embodiment includes an optical waveguide 30. Opticalwaveguide 30 is comprised of a source chamber 32, sample chamber 34 anddetector chamber 36. Source chamber 32 and detector chamber 36 arepreferably isolated from the sample chamber 34 by windows 38 and 39,which are interposed in the optical path between the source anddetector.

Top substrate 42 and bottom substrate 44 are micromachined from asemiconductor material using well known techniques so that when topsubstrate 42 is attached to bottom substrate 44 optical waveguide 30 isformed. Preferably substrates 42 and 44 are attached using a standarddie-attaching process.

Any semiconductor material may be used for substrates 42 and 44,including Si and GaAs. Si is the preferred semiconductor material.

Other semiconductor materials that may be used for the substrates of thepresent invention include binary, ternary, and quaternary II-VI andIII-V semiconductor materials. In addition, substrates consisting ofepitaxially grown germanium on silicon may be used.

Top substrate 42 and bottom substrate 44 may be made out of differentsemiconductor materials.

Windows 38 and 39, which have been micromachined in bottom substrate 44,divide optical waveguide 30 into source chamber 32, sample chamber 34,and detector chamber 36. Windows 38 and 39 thermally isolate the sourceand the detector from the gas sample being analyzed so that the gassample does not cool the detector or source, which would result in anerroneous reading, or at a minimum, reduced sensitivity. The thicknessof the windows is selected so as to achieve thermal isolation while atthe same time permitting the windows to be optically transparent as toat least desired wavelengths. That is, the thickness of the windows isselected to transmit at least a portion of the light having a desiredwavelength, the desired wavelength being dictated by the gas whoseconcentration is to be measured. As a result, windows 38 and 39 willtypically have a thickness in the range of approximately 0.1 mm to 1 mm.

Source chamber 32 contains a radiation source 40. In the presentembodiment, source 40 is a black body; thus, source 40 producesradiation of all wavelengths. Radiation from source chamber 32 passesthrough window 38 into sample chamber 34. Sample chamber 34 contains thegas sample to be analyzed for the concentration of a particular gaseouscomponent. Radiation entering sample chamber 34 passes through the gassample, and thereafter, the radiation enters detector chamber 36 throughwindow 39 and falls on detector 50 after passing through bandpass filter52.

FIG. 4 is a simplified optical diagram showing the optical path taken bya ray 46 emitted by the source 40. As illustrated, ray 46 is reflectedmultiple times as it progresses down the length of optical waveguide 30and eventually falls on the detector 50.

Referring back to FIGS. 1-3, bandpass filter 52 is preferably a narrowbandpass interference filter. The purpose of bandpass filter 52 is toremove extraneous wavelengths of radiation from the light produced byblack body source 40 so that the detector 50 primarily receivesradiation of a wavelength that is strongly absorbed by the gas whoseconcentration is to be determined.

In the present embodiment, bandpass filter 52 is a thin filminterference filter deposited on top of detector 50 and is, therefore,interposed in the optical path between source 40 and detector 50. Themethod of producing such filters is well known in the art. The theoryand physics of interference bandpass filters is described in detail inThe Infrared Handbook §§ 7.7-7.14 (W. Wolfe & G. Zissis Rev. ed. 3rdprinting 1989), hereby incorporated by reference.

The specific characteristics of the bandpass filter simply depends onthe wavelength of the radiation absorbed by the gas being analyzed. Forexample, a bandpass filter that has a center wavelength (CWL) of about4.26 μm and a full width at half maximum (FWHM) of about 0.1 μm isappropriate for CO₂.

As noted above, radiation source 40 in the present embodiment is a blackbody. FIG. 5 illustrates one possible configuration of a black bodylight source. Black body source 40 is comprised of a thin film resistiveheating element 67 deposited on cantilever 68. Thin film resistor 67 maybe made out of platinum or other suitable metal. Cantilever 68 isproduced using well known micromachining techniques. Such techniques arediscussed in K. Petersen, Silicon as a Mechanical Material, Proceedingsof the IEEE, Vol. 70, No. 5, May 1982, pp. 420-457, hereby incorporatedby reference.

Heating element 67 is placed on cantilever 68 to thermally Ads isolateit from substrate 44. Thermal isolation of heating element 67 isimportant so that the response time of black body source 40 is keptsmall and so that the entire substrate is not heated. In addition,cantilever 68 prevents heat produced by heating element 67 from drainingto substrate 44 too quickly.

Detector 50 may be any device that creates varying electrical responsesto corresponding variations in the strength of radiation reaching itfrom the source. Any of the following detectors may be used whenmeasuring infrared radiation: thermopile, thermocouple, PlatinumSilicide Schottky photodiode, and bolometer. A Thermopile is anespecially preferred detector for the detection of infrared radiation.

The more recent micromachined electron-tunneling infrared detectordisclosed in U.S. Pat. No. 5,265,470, issued Nov. 30, 1993, herebyincorporated by reference, can also be used in the present embodiment todetect infrared radiation.

Sample chamber 34 is a diffusion-type gas sample chamber. Thus, the gassample contained within sample chamber 34 is present by diffusion only,no pumps are required to push or pull the gas sample into sample chamber34. To permit gas to diffuse into and out of sample chamber 34, samplechamber 34 contains at least one filtering aperture, of which filteringaperture 54 is typical. The filtering apertures can have a wide varietyof shapes and sizes. However, it is preferable to prevent particles ofdust and smoke from entering sample chamber 34. To that end, Byfiltering apertures 54 are covered in the present embodiment with a gaspermeable layer 56, which serves as a filter. Gas permeable layer 56preferably keeps out particles of a size greater than about 0.1 μm.

Gas permeable layer 56 may be a dielectric layer that can be depositedby evaporation or sputtering. Silicon oxide, silicon nitride, TEFLON®,and KAPTON® are dielectrics suitable for this purpose, preferably thedielectric layer is silicon oxide or silicon nitride. TEFLON® is theregistered trademark for tetrafluoroethylene (TFE) fluorocarbonpolymers, and KAPTON® is the registered trademark for polyimide film.

To achieve high rates of diffusion for particles of size less than 0.1μm, gas permeable layer 56 should be kept quite thin, typically severalthousand angstroms. Thus, it is better to have a large number of smallerapertures than a few larger ones.

In the present embodiment, both the top and bottom substrates containapertures 54 permitting gas to diffuse into and out of the samplechamber. By having apertures 54 in both the top substrate 42 and bottomsubstrate 44, the gas sample flows more easily into and out of samplechamber 34, especially if the sensor 28 is placed in a flowing gasstream. Of course, however, the apertures may be contained in only thetop substrate 42 or bottom substrate 44.

Throat 57 in the top substrate 42 and throat 59 in the bottom substrate44 merely permit substrates 42 and 44 to have thicker cross sectionswhile at the same time reducing the amount of semiconductor materialthat must be etched through to form apertures 54 in walls 60 and 62 ofoptical waveguide 30. This enhances the overall structural integrity ofsensor 28 without increasing the difficulty of etching apertures 54 inthe substrates. Throats 57 and 59, however, are not required. Thinnerblocks of semiconductor material can be used to produce substrates 42and 44 without increasing the difficulty of etching holes in walls 60and 62 of the waveguide to form apertures 54.

For ease of manufacture, it is preferred that the sample chamber 34 havea square or rectangular cross section. But, other shapes can also beused. For example, sample chamber 34 in waveguide 30 can have a circularor triangular shape. Source chamber 32 and detector chamber 36 aresimilarly not limited to the preferred embodiment shapes.

If silicon is used as the semiconductor material for substrates 42 and44, only about half of the light will be reflected from the walls of theoptical waveguide because silicon is only about 50% reflective. Theefficiency of an NDIR detector according to the present embodiment maybe improved, therefore, by metallizing at least a portion of theinwardly facing walls of optical waveguide 30. Of course windows 38 and39 should not be metallized in such a manner as to prevent detector 50from receiving the wavelength of light that is strongly absorbed by thegas whose concentration is being measured. This would naturally defeatthe purpose of the waveguide. Thus, in the present embodiment, only theside walls 58, upper wall 60 and lower wall 62 of the sample chamber aremetallized. And in the source and detector chambers, only side walls 64and upper walls 66 are metallized.

A variety of different metallizations can be used to increase thereflectivity of the inwardly facing walls of optical waveguide 30,including Al, Au, Cr, Ni, and Pt. Each of these metals are easilydeposited on the surface of the inwardly facing walls of waveguide 30using vapor deposition techniques well known in the art.

To ensure that the full benefit of the metallization is achieved, themetallization must be thicker than the skin depth for the particularmetal selected at the wavelength being used to measure the concentrationof the desired gas. If the metallization is not sufficiently thick, theelectric field will penetrate the metallization and interact with the Sisubstrate; thus reducing the amount of light reflected.

In the present and subsequent embodiments, Applicant has described theuse of a top substrate 42 and a bottom substrate 44 to form gas sensor28. However, to the extent that gas sensor 28 is micromachined from asingle substrate, the resulting gas sensor would still fall within thescope of the present invention.

The NDIR gas sensor 28 can be used in a system such as the oneillustrated schematically in FIG. 6. The broken line 28 represents theNDIR gas sensor 28 shown in more detail in FIGS. 1-3.

In operation, source 40 is energized through an electric currentdelivered by source driver 76 through a circuit including leads 70. Asis well known in the art, source driver 76 delivers power to source 40in pulses at a set frequency to modulate the source so that synchronousdetection can be achieved. Typically, source driver 76 delivers pulsesof energy at a frequency of 1 Hz, 2 Hz, or 3 Hz.

After the radiation from source 40 reaches detector 50, detector 50produces an electrical signal that represents the intensity of theradiation falling on it. This signal is inversely proportional to theamount of gas being detected. For example, if CO₂ is being detected, themore CO₂ there is in sample chamber 34 the weaker the infrared spectrumwill be striking detector 50. The electrical signal produced by detector50 is applied to an electronic circuit 78 that converts it to a signalwhich represents the concentration of the gas in question. The signaloutput 80 of the signal processing electronics 78 may be applied to ameter, an alarm, process control means or the like as is well known inthe art.

The source driver 76 and signal processing electronics 78 are inelectronic communication through line 82 so that the entire system iscoordinated and synchronous detection can be achieved. The signalprocessing is generally straight forward and is taught in a number ofplaces, including, for example, U.S. Pat. No. 4,578,762, herebyincorporated by reference, and U.S. Pat. No. 4,694,173, herebyincorporated by reference.

In practice, a device embodying the present invention may be calibratedby placing it within a bell jar and purging the bell jar with nitrogen.When the bell jar has been thoroughly purged with nitrogen, the zerocondition for the sensor is established. Thereafter, a gas whoseconcentration will be measured by the sensor is fed into the bell jar ina gas stream of known concentration. For example, if CO₂ is the desiredgas then the bell jar would be purged with a gas having a CO₂concentration of 5% until the concentration of the gas within the belljar stabilizes. The signal processing electronics 78 are thenmanipulated until a reading of 5% is obtained from the sensor 28. Thisprocess is repeated using gases having different concentrations of CO₂until the NDIR gas sensor 28 is calibrated over the desired range ofconcentrations.

In the present embodiment, source 40 and detector 50 are both fabricatedon the same substrate. However, as would be apparent to a person ofordinary skill in the art, source 40 and detector 50 could be fabricatedon different substrates. But, by fabricating both source 40 and detector50 on the same substrate 44, the fabrication of the NDIR gas sensor 28is simplified.

A plan view of a bottom substrate 44 according to a second embodiment ofthe present invention is illustrated in FIG. 7. As shown in FIG. 7, ifsource 40 and detector 50 are both fabricated on substrate 44, thesource driver 76 and signal processing electronics 78 may also befabricated on substrate 44 near the source and detector using standardIC fabrication techniques. Substrate 44 is identical to substrate 44 ofthe first embodiment, which is illustrated in FIGS. 1-3, in all otherrespects.

Top substrate 42 is not shown. However, it is identical to the topsubstrate 42 shown in FIGS. 1-3.

By virtue of free diffusion of gas through layer 56 that covers thefiltering apertures 54, the gas sample chamber 34 is always filled withgas. As a result, there is a possibility that if the ambient temperaturefalls sufficiently, water vapor or one of the other gases in the samplechamber will condense to a liquid state and be deposited in the form ofsmall droplets on the interior walls of gas sample chamber 34. Thiswould interfere with the internal reflectivity that is needed foroperation of waveguide 30, and would lead to erroneous results.

To prevent this from happening, and thus increase the efficiency andrepeatability of the transmission of radiation, a particularly preferredbottom substrate 44 as shown in FIG. 8 may be used to fabricate an NDIRgas sensor 28 according to a third embodiment. As in the previousembodiment, top substrate 42 is not shown. Again, however, it isidentical to the top substrate 42 illustrated in FIGS. 1-3 so that whenit is attached to substrate 44, NDIR gas sensor 28 is formed.

As shown in FIG. 8, a thin film resistive type heater 86 is added to thetop of each boss 88. Bosses 88 have been micromachined into substrate 44and form the sidewalls of optical waveguide 30. Thin film resistancetemperature detectors (RTD's) 90 measure the temperature of sidewalls 58of the sample chamber 34. The heaters 86 and the thin film RTD's 90 areconnected to a temperature control circuit 92, which functions as aclosed loop controller that operates in a well-known way to maintain thesample chamber at a set temperature.

As with the source driver 76 and signal processing electronics 78, thetemperature control circuit 92 may be fabricated on substrate 44 usingstandard IC fabrication techniques.

Because the gas sensor of the present invention is fabricated from asemiconductor material such as silicon, other semiconductor integratedcircuit devices may also be added wherever needed to further enhance theperformance of the NDIR gas sensor 28 according to the presentinvention. For example, temperature sensors, pressure transducers, andhumidity sensors may be added to accomplish any of theknown in the art.In schemes known in the art. In addition, a micro-flow sensor may beadded to detect the flow rate of the sample gas through the samplechamber.

FIG. 10 shows a plan view of a bottom substrate 44 according to theembodiment illustrated in FIG. 8 with the addition of furtherperformance enhancing semiconductor devices manufactured thereon.Namely, a humidity sensor 100 has been added to the bottom substrate 44to monitor the humidity level in the sample chamber 32 during operationof the NDIR gas sensor. In the present embodiment, humidity sensor 100is a thin film capacitive humidity sensor. As illustrated in FIG. 11,bottom substrate 44 also has manufactured thereon, using techniques wellknown in the art, a pressure transducer 102 and a flow meter 104. Theoutput from these devices is communicated to the signal processingelectronics 78 to incorporate into the signal compensation schemes knownin the art.

A fourth embodiment of the present invention is described in connectionwith FIG. 9. This embodiment is preferred when the ability to detectmultiple gases is desired or when a reference detector is desired.

According to this embodiment, a bottom substrate 44 is provided. Bottomsubstrate 44 can have the basic features of any of the previousembodiments. However, instead of a single detector 50 and bandpassfilter 52 being fabricated on substrate 44, a plurality of detectors 50and bandpass filters 52 are fabricated on substrate 44 so that when topsubstrate 42, which has been omitted from FIG. 9, is attached they areall within the detector chamber 36.

Each bandpass filter 52 is interposed in the optical path between thelight source 40 and one of the plurality of detectors 50. The number ofgases whose concentration is desired to be determined dictates thespecific number of detectors 50 and bandpass filters 52 that arerequired.

Each bandpass filter 52 is designed so that the detector 50 it isassociated with primarily receives radiation of a wavelength that isstrongly absorbed by the gas whose concentration that specific detector50 is to determine. Thus, the illustrated substrate 44 can be used toform an NDIR gas sensor that can simultaneously determine theconcentration of a plurality of gases.

Of course, the source driver 76 and signal processing electronics 78 maybe added to substrate 44 in this embodiment. Similarly, if desired, theheaters 86, RTD's 90 and temperature control circuit 92 shown in FIG. 8may also be added to substrate 44.

Alternatively, if desired, in this embodiment one of the detectors 50shown on substrate 44 in FIG. 9 may be used as a reference detector. Thereference detector provides the signal processing electronics 78 with abaseline output signal against which to compare the output signal fromthe detector that is being used to measure the concentration of thedesired gas. As a result, the intrinsic stability of the sensor overtime may be improved because variations in the light intensity fromsource 40 are compensated for.

If one of the detectors 50 is to be used as a reference detector, thebandpass filter 52 associated with it must be designed so that thedetector primarily receives radiation of a neutral wavelength. In otherwords, the bandpass filter 52 must primarily pass radiation that is notabsorbed by the gas sample within sample chamber 34 and filter out otherextraneous wavelengths of radiation.

If there are more than two detectors within sample chamber 36, more thanone of them may be used as a reference detector. For example, if thereare three detectors 50 within detector chamber 36, two of them could beused as reference detectors, the bandpass filter 52 for each passing adifferent neutral wavelength. The third detector in this situation wouldbe used to determine the concentration of a desired gas.

Therefore, as would be apparent to one of ordinary skill in the art, theNDIR gas sensor 28 according to the present embodiment is very flexible.It may be used to measure the concentration of a plurality of differentgases, or it may be used to measure the concentration of one or moregases while simultaneously using at least one reference detector tomaintain the intrinsic stability of the sensor over time.

An NDIR gas sensor 108 according to a fifth embodiment is now describedin connection with FIGS. 12-15. As the present embodiment has many ofthe same features discussed in the previous embodiments, like referencenumbers have been used to refer to like features.

In the present embodiment, rather than locating radiation source 40 anddetector 50 at opposing ends of the sample chamber 34, the radiationsource 40 and detector 50 are provided at the same end of the samplechamber 34 in a side by side relationship. An advantage of thisrelationship as opposed to an NDIR gas sensor which has a straightoptical waveguide with a light source and a light detector provided atopposing ends is that the NDIR gas sensor 108 of the present embodimentis twice as sensitive. Thus, the NDIR gas sensor of the presentembodiment can achieve the same sensitivity with a sample chamber halfas long.

As shown in FIGS. 12-14, NDIR gas sensor 108 according to the presentembodiment includes an optical waveguide 30. Optical waveguide 30 iscomprised of source chamber 32, sample chamber 34, and detector chamber36. Source chamber 32 and detector chamber 36 are located on the sameend of optical waveguide 30, and sample chamber 34 is disposed in theoptical path between the two. Source chamber 32 and detector chamber 36are preferably isolated from the sample chamber 34 by windows 38 and 39,which are also interposed in the optical path between the source anddetector.

Top substrate 42 and bottom substrate 44 are micromachined from asemiconductor material using well known techniques so that when topsubstrate 42 is attached to bottom substrate 44 optical waveguide 30 isformed. Preferably substrate 42 and 44 are attached using adie-attaching process.

As with the first embodiment, any semiconductor material may be used forsubstrates 42 and 44, including Si and GaAs. Si, however, is thepreferred semiconductor material, and preferably substrates 42 and 44are made out of the same semiconductor material.

In the present embodiment, sample chamber 34 includes an optically openend defined by windows 38 and 39 and an optically closed end defined byend wall 110. Source 40 and detector 50 preferably occupy off-centerpositions at the optically open end of sample chamber 34. The end of thesample chamber defined by windows 38, 39 is referred to as beingoptically open because radiation of relevant wavelengths can passthrough windows 38 and 39. The end of the sample chamber 34 defined byend wall 110 is referred to as being optically closed because themajority of radiation striking end wall 110 from source 40 is reflectedback toward windows 38, 39.

Windows 38, 39 and divider 112, which have been micromachined in bottomsubstrate 44, divide optical waveguide 30 into source chamber 32, samplechamber 34, and detector chamber 36. Windows 38 and 39 thermally isolatethe source and the detector from the gas sample being analyzed so thatthe gas sample does not cool the detector or source, which would resultin an erroneous reading, or at a minimum, reduced sensitivity. As withthe first embodiment, the thickness of the windows should be selected soas to achieve thermal isolation while at the same time permitting thewindows to be optically transparent to desired wavelengths.

Divider 112 thermally isolates detector 50 from source 40. Divider 112should also have sufficient thickness to prevent light from source 40from entering detector chamber 36. Any such light transmittance wouldhave an obvious deleterious effect on the ability of the gas sensor 108to accurately measure the concentration of the gas to be detected. Toensure that no light is transmitted through divider 112 and that divider112 provides good thermal isolation, side walls 64 of divider 112 can bemetallized. Indeed, the efficiency of an NDIR gas sensor 108 accordingto the present embodiment can be improved by metallizing the walls ofoptical waveguide 30. Of course windows 38 and 39 should not bemetallized in such a manner as to prevent detector 50 from receiving thewavelength of light that is strongly absorbed by the gas whoseconcentration is to be measured. This would naturally defeat the purposethe waveguide. Thus, in the present embodiment, the side walls 58, upperwall 60, lower wall 62 and end wall 110 of the sample chamber aremetallized. And in the source and detector chambers, side walls 64 andupper wall 66 are metallized.

A variety of different metallizations can be used to increase thereflectivity of the walls of optical waveguide 30, including Al, Au, Cr,Ni, and Pt. Each of these metals are easily deposited on the surface ofthe inwardly facing walls of waveguide 30 using vapor depositiontechniques well known in the art.

To ensure that the full benefit of the metallization is achieved, themetallization must be thicker than the skin depth for the particularmetal selected at the wavelength being used to measure the concentrationof the desired gas. If the metallization is not sufficiently thick, theelectric field will penetrate the metallization and interact with the Sisubstrate; thus reducing the amount of light reflected.

Source chamber 32 contains a radiation source 40. In the presentembodiment, source 40 is a black body; thus, source 40 producesradiation of all wavelengths. In the simplest mode of propagation,radiation from source chamber 32 passes through window 38 into samplechamber 34, passes through the gas sample, reflects off end wall 110,passes through the gas a second time, and thereafter, the radiationenters detector chamber 36 through window 39 and falls on detector 50after passing through bandpass filter 52. An important advantage ofusing the NDIR gas sensor 108 of the present embodiment, however, isthat sample chamber 34 permits other modes of propagation from thesource to the detector to occur. The amount of radiation contributed bythe various modes of transmission is additive since the successive modesare characterized by progressively steeper angles of reflection of thelight rays. Further, the radiation travels a greater distance throughthe space within sample chamber 34 as the order of the mode ofpropagation increases. That is, for the higher modes, the angle ofreflection of the light rays is steeper resulting in a greater distanceof travel back and forth across the sample chamber 34, notwithstandingthat the distance traveled in the longitudinal direction remainsconstant and simply equals twice the length of the optical waveguide.

In the present embodiment, bandpass filter 52 is a thin filminterference filter deposited on top of detector 50 and is, therefore,interposed in the optical path between source 40 and detector 50.

The specific characteristics of the bandpass filter simply depends onthe wavelength of the radiation absorbed by the gas being analyzed. Forexample, a bandpass filter that has a center wavelength (CWL) of about4.26 μm and a full width at half maximum (FWHM) of about 0.1 μm isappropriate for C0₂.

As noted above, radiation source 40 in the present embodiment is a blackbody. FIG. 5 illustrates one possible configuration of a black bodylight source for the present embodiment.

Detector 50 may be any device that creates varying electrical responsesto corresponding variations in the strength of radiation reaching itfrom the source. Any of the following detectors may be used whenmeasuring infrared radiation: thermopile, thermocouple, PlatinumSilicide Schottky photodiode, and bolometer. A Thermopile is anespecially preferred detector for the detection of infrared radiation.

The more recent micromachined electron-tunneling infrared detectordisclosed in U.S. Pat. No. 5,265,470 can also be used in the presentembodiment to detect infrared radiation.

Sample chamber 34 is a diffusion-type gas sample chamber. To permit gasto diffuse into and out of sample chamber 34, sample chamber 34 containsat least one filtering aperture, of which filtering aperture 54 istypical. The filtering apertures can have a wide variety of shapes andsizes. However, it is preferable to prevent particles of dust and smokefrom entering sample chamber 34. To that end, filtering apertures 54 arecovered in the present embodiment with a gas permeable layer 56, whichserves as a filter. Gas permeable layer 56 preferably keeps outparticles of a size greater than about 0.1 μm.

Gas permeable layer 56 may be a dielectric layer that can be depositedby evaporation or sputtering. Silicon oxide, silicon nitride, TEFLON®,and KAPTON® are dielectrics suitable for this purpose, preferably thedielectric layer is silicon oxide or silicon nitride. TEFLON® is theregistered trademark for tetrafluoroethylene (TFE) fluorocarbonpolymers, and KAPTON® is the registered trademark for polyimide film.

To achieve high rates of diffusion for particles of size less than 0.1μm, gas permeable layer 56 should be kept quite thin, typically severalthousand angstroms. Thus, it is better to have a large number of smallerapertures than a few larger ones.

In the present embodiment, both the top and bottom substrates containapertures 54 permitting gas to diffuse into and out of the samplechamber. By having apertures 54 in both the top substrate 42 and bottomsubstrate 44, the gas sample flows more easily into and out of samplechamber 34, especially if the sensor 108 is placed in a flowing gasstream. Of course, however, the apertures may be contained in only thetop substrate 42 or bottom substrate 44.

Throat 57 in the top substrate 42 and throat 59 in the bottom substrate44 merely permit substrates 42 and 44 to have thicker cross sectionswhile at the same time reducing the amount of semiconductor materialthat must be etched through to form apertures 54 in walls 60 and 62 ofoptical waveguide 30. This enhances the overall structural integrity ofsensor 108 without increasing the difficulty of etching apertures 54 inthe substrates. Throats 57 and 59, however, are not required. Thinnerblocks of semiconductor material can be used to produce substrates 42and 44 without increasing the difficulty of etching holes in walls 60and 62 of the waveguide to form apertures 54.

For ease of manufacture, it is preferred that the sample chamber 34 havea rectangular cross section in the present embodiment. But, other shapescan also be used. For example, go sample chamber 34 in waveguide 30 canhave a circular or triangular shape. Source chamber 32 and detectorchamber 36 are similarly not limited to the preferred embodiment shapes.

The operation and calibration of NDIR gas sensor 108 according to thepresent embodiment is the same as that described in connection with theNDIR gas sensor 28 described in connection with FIGS. 1-6.

In the present embodiment, source 40 and detector 50 are both fabricatedon the same substrate. However, as would be apparent to a person ofordinary skill in the art, source 40 and detector 50 could be fabricatedon different substrates. But, by fabricating both source 40 and detector50 on the same substrate 44, the fabrication of the NDIR gas sensor 108is simplified.

As illustrated in connection with FIG. 15, source driver 76 and signalprocessing electronics 78 may also be fabricated on substrate 44 nearthe source and detector using standard IC fabrication techniques. Theperformance of the NDIR gas sensor 108 according to the presentembodiment can also be improved by adding a thin film resistive typeheater 86 and thin film RTD's 90 to the top of each boss 88 to preventmoisture from condensing on the interior walls of sample chamber 34.Operation of these devices was previously described in connection withFIG. 8.

Other semiconductor integrated circuit devices may also be addedwherever needed to further enhance the performance of the NDIR gassensor 108 according to the present invention. For example, temperaturesensors, pressure transducers, and humidity sensors may be added toaccomplish any of the signal processing schemes known in the art.

A sixth embodiment of the present invention is now described inconnection with FIGS. 16 and 17.

Certain gases have absorption bands that absorb so weakly thatabsorption can only be detected after the radiation has traveled arelatively long distance, perhaps tens of meters, through the samplegas. In other situations, the gases of interest might have adequateabsorption strengths but they must be detected in very lowconcentrations, typically in parts per million (ppm) or less, so thatlong path lengths are required in this situation also.

In the various embodiments of the invention described in connection withFIGS. 1 through 11, a straight sample chamber 34 was employed. By usinga straight sample chamber 34, however, the length of the opticalwaveguide 30, and thus the sensitivity of the NDIR gas sensor 28, isphysically limited by the length of the substrate 44. As a result, theNDIR gas sensors described in connection with those figures may haveinsufficient path lengths to adequately monitor the concentration ofcertain gases. In the present embodiment, as illustrated schematicallyin FIGS. 16 and 17, an optical waveguide is employed that includes anextended sample chamber 114 that extends along an indirect path.Preferably the indirect path traversed by the extended sample chamber114 according to the present embodiment has a serpentine shape asillustrated in FIG. 16 or a spiral shape as illustrated in FIG. 17. Theremaining features and possible modifications of the NDIR gas sensor 28according to the present embodiment are identical to those described inconnection with the embodiments illustrated in FIGS. 1-11. Thus, as withthe embodiments described in connection with FIGS. 1-11, the lightsource 40 and light detector 50 of the present NDIR gas sensor arelocated at opposite ends of the optical waveguide 30, and a pair ofwindows 38, 39 are preferably disposed between the light source anddetector so as to define a sample chamber therebetween.

An advantage of the present embodiment is that a sample chamber having apath length much longer than the length of the substrate in which it isformed can be produced, thus providing enhanced sensitivity to the NDIRgas sensor without substantially increasing its overall dimensions.

The walls of the extended sample chamber 114 preferably have a mirrorlike finish. This can be accomplished by metallizing the walls of thesample-chamber with a metal such as Al, Au, Cr, Ni, or Pt. As a resultof its specularly reflective surface, the radiation that has beendirected into the extended sample chamber 114 will progress bysuccessive reflections through the sample chamber, and a fraction of theradiation will reach the other end of the sample chamber 114, enterdetector chamber 36 through window 39, and eventually fall on detector50 after passing through bandpass filter 52.

A number of apertures, such as filtering apertures 54 shown in FIGS. 2,and 7-9, permit communication between the extended sample chamber 114and the air in the space surrounding the gas sample chamber. The gaswhose concentration is to be measured freely enters and exits theextended sample chamber 114 by diffusion through these passages.

The longer the extended sample chamber 114 is, the less radiation thatwill reach the detector end of the optical waveguide 30. As a result,there is a trade-off between the diminishing intensity that results asthe path length is made longer against the increased absorption that isavailable with a longer path length. Thus, when determining theappropriate path length for the sample chamber 114 for an NDIR gassensor according to the present embodiment, the following factors shouldbe taken into account: (1) the particular gas to be detected, (2) theintensity of the source 40, and (3) the sensitivity of the detector 50.

Operation and calibration of an NDIR gas sensor 28 employing an extendedsample gas chamber 114 is much the same as that described in connectionwith the embodiment illustrated in FIGS. 1-11.

While the present invention has been made clear in the illustrativeembodiments, it will be immediately obvious to those skilled in the artthat many modifications of structure, arrangement, proportions, theelements, materials, and components used in the practice of theinvention, and otherwise, which are particularly adapted to specificenvironments and operative requirements, can be made without departingfrom the principles disclosed. Thus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of the invention as claimed below.

I claim:
 1. An improved NDIR gas sensor, comprising:a. a firstsemiconductor material substrate; b. a second semiconductor materialsubstrate attached to the first substrate; c. an optical waveguideformed from the first and second substrates; d. a light sourceintegrally formed on one of the substrates and positioned at one end ofthe optical waveguide; e. a light detector integrally formed on one ofthe substrates and positioned at the end of the optical waveguideopposite the light source; f. a bandpass filter interposed in theoptical path between the light source and light detector; g. a pair ofsemiconductor material windows interposed in the optical path betweenthe light source and light detector so as to define therebetween asample chamber within the waveguide and thermally isolate the lightsource and light detector, wherein the sample chamber extends along anindirect path; and h. means for permitting diffusion of gas into and outof the sample chamber.
 2. An NDIR gas sensor according to claim 1,wherein the light source and light detector are fabricated on the samesubstrate.
 3. An NDIR gas sensor according to claim 1, wherein at leasta portion of the walls of the optical waveguide are metallized toimprove internal reflectivity of the waveguide.
 4. An NDIR gas sensoraccording to claim 1, further comprising means for heating the walls ofthe optical waveguide to a temperature above the dew point of the gas inthe sample chamber.
 5. An NDIR gas sensor according to claim 1, furthercomprising means for sensing the humidity level in the optical waveguidesample chamber.
 6. An NDIR gas sensor according to claim 1, furthercomprising means for detecting the pressure level within the opticalwaveguide sample chamber.
 7. An NDIR gas sensor according to claim 1,where in the light detector is an infrared detector selected from thegroup consisting of Platinum Silicide Schottky photodiode, bolometer,infrared tunnel sensor, thermocouple, and thermopile.
 8. An NDIR gassensor according to claim 1, further comprising:a. a source driver forenergizing the source at a preset frequency; and b. signal processingelectronics for converting the electrical output produced by thedetector into a signal which represents the concentration of the gasbeing measured.
 9. An NDIR gas sensor according to claim 1, wherein themeans for permitting gas to diffuse into and out of the space within thesample chamber comprises at least one aperture in a wall of the samplechamber.
 10. An NDIR gas sensor according to claim 1, wherein the meansfor permitting gas to diffuse into and out of the sample chambercomprises:a. a plurality of apertures in at least one wall of the samplechamber; and b. a gas permeable layer covering the plurality ofapertures.
 11. An NDIR gas sensor according to claim 10, wherein the gaspermeable layer is made from a dielectric selected from the groupconsisting of TFE fluorocarbon polymers, polyimide, silicon oxide, andsilicon nitride.
 12. An NDIR gas sensor according to claim 10, whereinthe gas permeable layer prevents particles larger than 0.1 μm fromentering the sample chamber within the optical waveguide.
 13. An NDIRgas sensor according to claim 1, wherein the pair of windows aremicromachined from one of the semiconductor material substrates.
 14. AnNDIR gas sensor according to claim 1, further comprising:a. a pluralityof light detectors integrally formed on one of the substrates andpositioned at the end of the optical waveguide opposite the lightsource; b. a plurality of bandpass filters, wherein each bandpass filterpasses a different range of wavelengths and is interposed between thelight source and one of the plurality of light detectors.
 15. An NDIRgas sensor according to claim 14, wherein at least one of the pluralityof bandpass filters is designed to pass a neutral wavelength.
 16. Animproved NDIR gas sensor, comprising:a. a first semiconductor materialsubstrate; b. a second semiconductor material substrate attached to thefirst substrate; c. an optical waveguide formed from the first andsecond substrates; d. a light source integrally formed on one of thesubstrates and positioned at one end of the optical waveguide; e. alight detector integrally formed on one of the substrates and positionedat the end of the optical waveguide opposite the light source; f. abandpass filter interposed in the optical path between the light sourceand light detector; g. a first and a second semiconductor materialwindow disposed in the waveguide between the light source and lightdetector; h. a sample chamber interposed between the first and secondwindows in the optical waveguide, the sample chamber extending along anindirect path between the first and second windows; i. at least oneaperture in the wall of the sample chamber.
 17. An improved NDIR gassensor, comprising:a. a first silicon substrate; b. a second siliconsubstrate attached to the first silicon substrate; c. an opticalwaveguide formed from the first and second silicon substrates; d. ablack body light source integrally formed on the first silicon substrateand positioned at one end of the optical waveguide; e. an infrared lightdetector integrally formed on the first silicon substrate and positionedat the end of the optical waveguide opposite the light source; f. abandpass filter interposed in the optical path between the light sourceand light detector; g. a pair of optically transparent silicon windowsinterposed in the optical path between the light source and lightdetector so as to define therebetween a sample chamber within thewaveguide and thermally isolate the light source and light detector fromthe sample gas, the sample chamber extending along an indirect pathbetween the light source and light detector; h. a plurality of aperturesin at least one wall of the sample chamber; i. a gas permeable layercovering the plurality of apertures to prevent particles larger than 0.1μm from entering the sample chamber; j. a source driver for energizingthe source at a preset frequency; and k. signal processing electronicsfor converting the electrical output produced by the detector into asignal which represents the concentration of the gas being measured. 18.An improved NDIR gas sensor, comprising:a. a semiconductor materialsubstrate; b. an optical waveguide formed within the substrate, theoptical waveguide comprising a source chamber, a detector chamber and asample chamber, the source and detector chambers separated from thesample chamber by a first and a second semiconductor material window,respectively, and the source and detector chambers being located on thesame end of the optical waveguide; c. a light source integrally formedon the substrate in the source chamber; d. a light detector integrallyformed on the substrate in the detector chamber; e. a bandpass filterinterposed in the optical path between the light source and lightdetector; f. at least one aperture in a wall of the sample chamber,wherein light from the source is reflected off at least one wall of thesample chamber before reaching the detector.
 19. An improved NDIR gassensor, comprising:a. a first semiconductor material substrate; b. asecond semiconductor material substrate attached to the first substrate;c. an optical waveguide formed from the first and second substrates, theoptical waveguide comprising a source chamber, a detector chamber and asample chamber, the source and detector chambers separated from thesample chamber by a first and second semiconductor material window,respectively, and the source and detector chambers being located on thesame end of the optical waveguide; d. a light source integrally formedon one of the substrates in the source chamber; e. a light detectorintegrally formed on one of the substrates in the detector chamber; f. abandpass filter interposed in the optical path between the light sourceand light detector; g. at least one aperture in a wall of the samplechamber, wherein light from the source is reflected off at least onewall of the sample chamber before reaching the detector.